EEVblog #538 - HP35670A DSA Repair - Part 2

[Rasz]

Comparison of Vimeo vs Youtube as I saw it.

use this to compare
http://www.freedownloadmanager.org/download/MSU_Video_Quality_Measurement_Tool_19505_p/

comparing on feel is so audiofool

[envisionelec]

I'm going out on a limb and saying that it's not the ASIC and it's probably not unobtanium. Too bad there seems to be no available schematics.  If I was really driven to succeed, I'd build a jumper/jig to get the board out of the chassis for testing.

[vaualbus]

Apparenty it seem that the system use an ADC and the atmel asic (fpga) to controll it. You can see a block diagram of the unit in service manual.
If it so probably the ADC is near by the fpga. If so we are a little bit more lucky given the fact that the asic if is only nasted logic work at 5V at least.
Apparently in the circuit there some dac to introduce a voltage to reduce the adc noise!,a +/- 6.3 reference adc voltage is provided by a dac controlled by the asic (fpga).
So I'll begin to look for that reference voltages.
Second I think that one of the two transistor on the attenuator board is a step up dc - dc converter to generate the test voltages  for the system, try to check also that two transistor.
That as I said on youtube try to see If it it's possible to adjust the -19.2 psu rail as close as possible to -18v the corret voltage.
Check also the back board because aparently also that board use the -18 rail.
The strange think is the i2c error becuase in the manul there no reference on what can cause that error.

[stranger]

Thought on the Atmel FPGA / ASIC, some good progress with the PSU and yes that raises a suspicion re the Atmel part but there is a chance the analogue ‘asic’ error is not related. The memory that finally crawled to the front of my mind was with similar socked parts and is worth a check, easy fix if you are lucky. Dendrites (whiskers) growing under the socket and providing shorts between adjacent pins, fix using a blast of IPA and a small stiff brush poked between pins under the socket being enough to clear some faults in the past.

http://www.purafil.com/PDFs/Technical%20Papers/Industrial/Corrosion%20The%20Unseen%20Enemy.pdf

http://en.wikipedia.org/wiki/Dendrite_(metal)

A real pain is when it happens on an internal PCB layer, 

[Stonent]

Actel. not Atmel.

[os40la]

Dave,

Do you have this Agilent 35670A Service Guide?

http://cp.literature.agilent.com/litweb/pdf/35670-90066.pdf

[NiHaoMike]

It's highly unlikely for an overvoltage to take out the 4148. That would take more than 100V, at which point there would be a lot of burn marks everywhere. Most likely the rail shorted and applied reverse bias for just a moment.

[EEVblog]

Do you have this Agilent 35670A Service Guide?

Yes, of course, but there are no schematics.

[84GKSIG]

silly but I,m hanging on the edge of my seat to see this thing work

[TopGunPk]

Hi Dave. Not sure if this could be anything, but mostly I2C EEPROMS are used to store CAL data and constants. Some would remain same over the years, some might change depending on a lot of factors.

Your 5V (digital) rail is OK, so its unlikely the I2C EEPROM has failed, however have you considered the possibility that when the original PSU went kaput, it might have caused a high spike or kickback, and probably that was enough to either : a) destroy the I2C EEPROM itself, OR b) corrupt / destroy some of the CAL data on it ?

From a software point of view, the software would start by getting the CAL data from the I2C EEPROM and then start reading the ADC, presumably by connecting one or all channels to known reference voltages to perform calibration. Most of the times, you need to have the previous CAL data (and that too within close limits) to get that CAL function to return a proper value. Otherwise, if the return values are not within a specific limit, it MIGHT just assume that the Analog or ADC section has failed !

Just a hunch.

[pickle9000]

My take on this repair is... very entertaining, I love troubleshooting.

- I'd pop out the eeprom and make a copy, mainly so I could have a starting point if needed later. It's good standard practice when possible.
 
- I like the fact that Dave is taking money into account, any repair can be done given enough time but "Is it economical for the customer?". I have no problem doing things purely for enjoyment or education but in a real job always think of the customer. Skipping the manual and checking the other regulators also says he is keeping the customer in mind. A fast repair save the customer money and you get to make more from them in the future.
 
- A very professional way of checking the temp of a chip. Use the back of the finger, it's sensitive and if you get a burn you can keep on working. Very "old school"

[ddavidebor]

Yep that's right!!!  Call data loss usually give adc error! And analog frontend error also!!!

[F4CPY]

According to the 4-15 part and 10-3 section, what code is displayed on the leds during a time of more than 4 sec?

[kaushleshchandel]

If you succeed, I may go for buying it and trying to repair it.

It always is like a state lottery. The only difference is sometimes you can actually win.

I agree its a lottery. Dave's video is a good step by step process on trying a repair. Even getting the Datron 9000 from "no power" to "power up with error" would be an achievement for me. I have never ever tried a repair on a large instrument like that.

[berniwa]

Hey Dave,

generally i think it would be an idea to identify what may have caused the fault.

In my opinion the diode and vreg damage indicates a positive voltage on the -18V supply or a voltage spike. Most IC have internal clamping diode so if the negative supply goes positive this diodes will conduct to gnd and you will get a current flow accros the little input diode you replaced. If I'm correct you may see some of the clamping diodeson the IC pins damage (having different reverse voltages) or dead (be carefull with Analog Pins, which may not have clamping diodes).

In this case the device may be beyond economical repair.

Greets
Berni

[Winston]

I wonder if the original power supply was taken out by a lightning strike.

A $20k unit plugged into an unprotected socket... seeing that my sub-1k gear is on a UPS, this thought makes me cringe.

I've seen far more expensive units than that plugged into unprotected sockets at several different universities and numerable military units, especially the mobile ones. This gets back to the original point of my post - my surprise that HP designs or allows a subcontractor to design $1500 power supplies that take out an entire unit when they fail or allow the power supply to be taken out by the unit after it's replaced due to inadequate output protection.

[LaurenceW]

I'm with BKTemp on this one. Maybe the negative 18V supply failed in the original power supply. The remaining circuitry in the instrument then attempts to put a reverse (i.e. positive) voltage across the negative regulators, and this will take them and any "piss-amp" (must be an ozzy phrase?) diodes with them in a flash. I've done that very thing myself!  Fault intolerant design, in my case.

I'd be VERY surprised if a raw -18V supply reached op amps etc. designed to run at -15V, would cause them any embarressment at all. But -18V onto any rail designed to run at -5V... that could do some damage.

I still need convincing that all of the cards are now carrying the appropriate main power supply voltages, and locally regulated on-card voltages.

[EEVblog]

I still need convincing that all of the cards are now carrying the appropriate main power supply voltages, and locally regulated on-card voltages.

I thought I showed that in the video? All rails on all cards are now good, unless I missed one.
Next video has been shot and uploaded for release. Subscribers have already seen it 

[baljemmett]

this will take them and any "piss-amp" (must be an ozzy phrase?) diodes with them in a flash

"Pissant", meaning insignificant

[vaualbus]

Dave when you release the next video on youtube? By the way you could ask to agilent for the schematics an
saying that you do a video on that. Pheraps agilent give the schematics. I was luck and optained the true schematics for my tektronix tds540 by asking to tek the schematics. Wait for the public release of the video. Best regards Alberto.
Also if you are subscribed there more video or you just see the new video before?

[Galaxyrise]

You showed a chart of measured power supply voltages, and that got me thinking.  When checking a "maybe bad" power supply, when do you check with a scope and not just a DMM?

[Stonent]

Dave when you release the next video on youtube? By the way you could ask to agilent for the schematics an
saying that you do a video on that. Pheraps agilent give the schematics. I was luck and optained the true schematics for my tektronix tds540 by asking to tek the schematics. Wait for the public release of the video. Best regards Alberto.
Also if you are subscribed there more video or you just see the new video before?

Well he did on their forum, but that was towards the time of the first video, they redirected him to the PSU area and he got no responses.

[vaualbus]

I don't mean by ask to the forum, I was in the situation on tek forum, but directly to the agilent help services.

[EEVblog]

Well he did on their forum, but that was towards the time of the first video, they redirected him to the PSU area and he got no responses.

Why they moved it to the PSU area just baffles me.
But in any case, it looks like the schematics are unobtainium.

[vaualbus]

This is bad but I wish you'll able to fix it.
Although have you tried to put a signal in the dsa?